Twenty-five percent of the monkeys, Aotus trivirgatus 13/52 and Saimiri sciureus 5/15, inoculated intracerebrally with JC virus developed cerebral astrocytomas. The induction period for the tumors was 14 - 30 months following inoculation. The astrocytomas were similar in both species of monkeys. All showed high cellularity, mitotic figures and cellular pleomorphism. JC T antigen antibody titers developed in the inoculated monkeys but did not correlate with tumor production.